Solid-state lighting devices made of light-emitting diodes are increasingly useful for applications requiring robustness and long-life. Organic light-emitting diode solid-state lighting devices are of great interest because they may provide a light-emitting area rather than a point source of light (as is found in inorganic light-emitting diodes). This attribute of organic light-emitting diode lighting devices reduces the maximum heat of the device and facilitates efficient extraction of light from the diodes in a lighting fixture or lumiere.
Organic light emitting diodes (OLEDs) can be manufactured by depositing materials on a substrate and encapsulating them with a cover or layer. This process enables the creation of single, monolithic lighting elements on a single substrate. For example, FIG. 2 illustrates an OLED device suitable for lighting. Referring to FIG. 2, a prior-art OLED solid-state lighting device 10 may comprise a substrate 102 with a first electrode 104 deposited thereon. One or more layers of organic material 106 may be deposited on the first electrode 104, as is known in the prior art. A second electrode 108 is formed over the one or more layers of organic material 106. An encapsulating cover 110 protects the electrodes 104 and 108 and the one or more layers of organic material 106. The encapsulating cover 110 may also be a layer coated over the second electrode 108 (not shown). The electrodes 104 and 108 pass beneath the encapsulation layer 110 and are exposed as connectors to an external power source 112. The connectors may be arranged in a variety of configurations and can include, for example, insulating layers 114. In operation, when the external power source 112 supplies a voltage to the electrodes 104 and 108, a current passes through the one or more layers of organic material 106 and causes the organic materials to emit light.
OLED devices are subject to a variety of manufacturing difficulties and are also subject to changes in performance as the OLEDs are used. For example, it can be difficult to maintain a consistent performance from one device to another, even when the devices are manufactured on a common substrate or in a common process. Likewise, as is well known, OLED devices become less efficient over time and output less light with the same amount of power.
Since the brightness of an OLED lamp depends on the magnitude of the applied voltage, a variety of techniques have been used to compensate for aging by increasing the voltage applied to the lamp as the lamp ages. The simplest control is a dimmer, with which one manually adjusts the brightness of the lamp to suit one's taste. For EL (Electroluminescent) lamps powered from an AC power source, a dimmer is simply a potentiometer in series with the lamp. Most EL lamps are driven by an inverter powered by a battery. The inverter typically includes a switching circuit that produces a series of pulses which are coupled to the lamp. The switching circuit can be self-oscillating or driven. Dimming inverters are known in the art and typically use pulse width modulation to change the voltage across the EL lamp.
Both manufacturing variability and OLED aging have been addressed in the prior art. For example, U.S. Pat. No. 6,353,291 by Borgogno et al. entitled “Electroluminescent lamp controller” describes electronic sensing circuits that monitor multiple sized EL cells, referred to as a lamp, and compensate for variations in contrast between cells, while maintaining a fixed contrast between the luminance of the cells and the ambient lighting. In one embodiment, three separate feedback loops monitor the ambient lighting, cell-luminance, and frequency of the excitation voltage and make appropriate adjustments to an adjustable luminance reference. In another embodiment the circuit that drives the EL cells includes a timer and a microprocessor. The timer measures the elapsed time during which the EL display has been operating. The microprocessor adjusts the drive signal to the EL display to correct for aging, based on the elapsed time measured by the timer and an empirically-determined aging parameter. All these adjustments are accomplished while a microprocessor sequences through a large assortment of electroluminescent cells of various sizes.
In a related approach, U.S. Pat. No. 5,493,183 by Kimball entitled “Open loop brightness control for EL lamp” discloses an EL lamp characterized by non-linear brightness decay that is driven to constant brightness for the life of the lamp by an inverter controlled by a microprocessor. The microprocessor controls the inverter in accordance with a table containing data that is the inverse function of the non-linear brightness decay. The microprocessor tracks the total on-time of the EL lamp and selects the appropriate data from the table for driving the lamp. Control can be analog or digital. If control is analog, the selected data is coupled to a D/A converter which is coupled to the inverter. If control is digital, the selected data is coupled to a counter driving a pulse width modulator which is coupled to the inverter. In accordance with another aspect of the invention, an EL lamp is dimmed a precise amount based upon the data in the table and the EL lamp can be dimmed in unison with other lamp types, each lamp type having a table containing data for dimming. However, such designs require expensive controls (e.g. a microprocess) for tracking device use and providing corrective control.
Automatic brightness controls include optical feedback, in which a photocell monitors lamp brightness and varies the applied voltage to obtain a predetermined brightness, and analogue feedback, in which a lamp property, such as capacitance, is monitored to determine the applied voltage. U.S. Pat. No. 5,089,748 by Ihms entitled Photo-feedback Drive System discloses a system including optical feedback for maintaining the brightness of an EL lamp. U.S. Pat. No. 6,337,541 by Dickie et al. entitled “Electroluminescent lighting device” describes a supplementary lighting device with a replaceable electroluminescent light element. A control system is provided to boost the light intensity of the electroluminescent light element. A light sensor is provided which detects light emitted from the electroluminescent lighting element and inputs this information to the control system. The control system automatically adjusts the intensity of the electroluminescent lighting element according to a pre-set value and the information from the light sensor. This allows for the electroluminescent lighting element to maintain a constant output despite the ageing of the electroluminescent lighting element. The pre-set value for the intensity of the electroluminescent lighting element can be adjusted by a user-controlled dimmer. The dimmer varies the amount of light detected by the light sensor in order to increase or decrease the power provided by the control system to the electroluminescent lighting element.
Such a design requires an expensive optical sensor and the location of the sensor makes designing a display difficult because the sensor must be hidden and one must make separate connections to the sensor and to the lamp. Unless precision sensors are used, the part-to-part variability in sensors may cause unacceptable results. Moreover, such optical feedback designs require a large number of additional and expensive components and are subject to error from ambient light.
It is also important that lighting devices in common use by consumers be compatible with the existing lighting infrastructure, for example the common screw-type base known in the United States and the pin-type bayonet bases used in Europe. Moreover, the lighting devices must be readily and safely replaced by consumers at minimal cost.
In view of the foregoing, it is therefore an object of the invention to provide a low cost system for maintaining the brightness of an OLED lamp as it ages. A further object of the invention is to provide a control system that uses minimal additional and inexpensive components, and has a simple construction, low cost, and compatibility with the existing lighting infrastructure.